Background: OFDM (Orthogonal Frequency-Division Multiplexing) combats frequency-selective multipath fading — where different frequencies in the signal fade differently — by splitting a wideband channel into many narrowband, mutually orthogonal subcarriers, each of which sees an approximately flat channel gain. Every OFDM symbol is prefixed with a cyclic prefix (CP) of duration Tcp: a copy of the tail of the symbol, pasted onto its front. As long as every multipath tap arrives within Tcp, the CP absorbs the channel's delay spread, so the receiver can equalize each subcarrier with a single complex multiply and no inter-symbol interference (ISI, energy leaking from the previous symbol) or inter-carrier interference (ICI, loss of orthogonality between subcarriers) occurs. If Tcp is too short for the channel's delay spread, the taps that arrive after Tcp are not fully captured by the receiver's FFT window, degrading the effective SINR (signal-to-interference-plus-noise ratio) and producing a visibly noisier constellation and higher bit error rate (BER).
Description of This Web Application: Choose an LTE channel bandwidth, subcarrier spacing, modulation order (QPSK/16-QAM/64-QAM), and SNR, and watch a simulated 4096-bit packet travel from a base station to a mobile device every 2 seconds — representing how often we re-observe the channel, not how long a packet takes to transmit. The app shows the transmitted spectrum and time-domain waveform, a randomly generated frequency-selective multipath channel with six taps at fixed delays τ0…τ5 (its impulse response and frequency response), the received waveform after the channel and noise, and the transmitted versus received constellations with the resulting BER. Any tap that arrives after the (fixed) cyclic prefix is flagged in red on the impulse response and contributes real ISI/ICI power to the link. You will learn how frequency selectivity distorts different subcarriers differently, and how modulation order trades data rate for noise robustness.